1. Field of the Invention
This invention relates to the field of gas gauges for use in vacuum or ambient environments.
2. Related Art
A typical air gauge is very sensitive to environmental effects, such as the pressure, humidity, and temperature of the air. One method of canceling out these effects is to use a reference nozzle and a measurement nozzle. The measurement nozzle uses air flow pressure differences to measure a distance between the nozzle and a surface. For example, the surface may be a semiconductor wafer or LCD panel. The reference nozzle discharges air toward a reference plate. The surface of the reference plate is located at a pre-determined distance from the reference nozzle. The distance of the measurement nozzle to the surface can be determined by comparing the difference between the reference air flow pressure and the measurement airflow pressure.
If the properties of environmental effects are uniform over a large area, or if the measurement is low-resolution so that minor fluctuations in the environment do not affect the measurement, the same environment should exist at both the reference and measurement nozzles. These effects can be canceled out relatively easily. However, this process becomes ineffective when high-resolution measurements, such as those on the order of nanometers, are required. When a small measurement is needed, local environmental differences between the measurement nozzle and the reference nozzle significantly affect the measurements, even when the nozzles are only a centimeter or two apart. If the environmental differences are variable and changing with time, a variable offset results that causes an unpredictable measurement error.
For example, if there is a net air flow from one nozzle to the other, a pressure difference exists between the two. If that air flow changes, the pressure difference changes, resulting in an inaccurate measurement.
Measurement errors resulting from variable environments are not predictable. This is important in many applications, such as lithography. In lithography, movement of a wafer stage to different positions dramatically affects the local air flow. Thus, offset caused by pressure differences is motion or velocity dependent rather than fixed on the wafer. What is needed is a gauge that remains unaffected by changes in the local environment.